The general technical problem to be solved by any device for measuring of the speed of moving organs and blood flows is to obtain an exact as possible estimate of the axial speed of the movement being studied in order to form, using imaging devices, exact images of the organs and the blood flows studied by ultrasonic echographic scanning.
Various solutions to this problem have already been proposed. For example, European Patent Application No. 0 225 667 which corresponds to U.S. Pat. No. 4,885,990 describes such a device the measuring the speed of moving organs and blood flows
which are backscattered by a moving target are inked by the following equation when the transmission is recurrent with a recurrent period T: EQU S.sub.n+1 (t)=S.sub.n (t-.tau.). (1)
This means that the signal n+1 is the replica of the preceding signal n, except for a time shift .tau.. The latter represents the additional time necessary for the ultrasonic wave to follow the path between the transducer, the target and the transducer from one activation to the next. In other words: EQU .tau.=2VT/C
where V is the speed of the target and C is the speed of sound. It appears that measurement of .tau. enables measurement of the speed V.
The intercorrelation function between S.sub.n (t) and S.sub.n+1 (t) defined by: ##EQU1## verifies that: EQU C.sub.n,n+1 (to,u)=C.sub.nn (to, u-.tau.)
The time to is linked to the scanning depth z as to=2z/C, and W is the integration window.
The function C.sub.nn (to, u) is an autocorrelation function and is, therefore, maximum for u=o. Thus, the time shift .tau. and hence the speed V can be measured by searching the parameter u for which the function C.sub.n, n+1 (to, u) is maximum. Therefore, the intercorrelation function is sampled with a sampling step .DELTA.t, between u.sub.min =-I.DELTA.t and u.sub.max =I.DELTA.t in steps of 1 so as to obtain 2I+1 correlation function values. The maximum value of these 2I+1 values, corresponding to u=uo, enables measurement of .tau. by utilizing the equality .tau.=uo.
In order to eliminate errors inherent of the sampling during the determination of the maximum of the correlation function, use can be made of a multiplexing/interpolation circuit which supplies a more exact estimate of the speed and the corresponding peak value on the basis of correlation function values. French Patent Application No. 2 590 790 which also corresponds to U.S. Pat. No. 4,803,990 describes an example of this type of echographic signal processing where the correlation between signals is a so-called "1-bit" correlation in a sense that the signals S.sub.n+1 and S.sub.n previously used are reduced to the sign of the ultrasonic signal. It is known that in that case the correlation function peak is shaped as an isosceles triangle. Knowledge of this shape enables complete reconstruction of the correlation peak, starting from the highest point and its two neighbors, and using linear interpolation, and hence exact determination of the position of uo.
This known method for the measurement of speeds, based on the analysis of the time shift, has substantial advantages over other methods which are based, for example on the frequency or phase shift. It notably enables the use of wideband transmission signals offering a suitable axial resolution of the measurement.
However, the method described above does not enable measurement of speeds higher than a speed limit V.sub.lim which is given by: ##EQU2## where C represents the propagation speed of the ultrasonic wave. This phenomenon, also known as "aliasing", is linked to the indetermination induced by the periodicity of the echographic signal. A detailed description thereof is given in "Doppler Ultrasound and Its Use in Clinical Measurement", P. Atkinson and J. P. Woodcock, Academic Press, 1982.
For example, for a recurrent period T of 100 .mu.s a central acoustic frequency f.sub.n of 5 MHz, and a propagation speed C of 1500 m/s, a limit speed V.sub.lim of 75 cm/s is obtained, whereas, for example, given blood flows can reach speeds which are substantially higher.
In order to increase the measuring limit speed, it could be contemplated to reduce the frequency f.sub.0, but that would lead to a reduction of the precision of measurement and the resolution. Likewise, an increase of the recurrent frequency would have the undesirable effect of a decreased scanning depth.